This invention generally relates to the field of communication systems and, more particularly, to digital communication systems that supports multiple modulation and channel coding schemes.
In wireless digital communication systems, standardized air interfaces specify most of system parameters, including modulation scheme, channel coding scheme, burst format, communication protocol, symbol rate, etc. For example, European Telecommunication Standard Institute (ETSI) has specified a Global System for Mobile Communication (GSM) standard that uses time division multiple access (TDMA) to communicate control, voice and data information over radio frequency (RF) physical channels or links using Gaussian Minimum Shift Keying (GMSK) modulation scheme at a symbol rate of 271 ksps. In the U.S., Telecommunication Industry Association (TIA) has published a number of Interim Standards, such as IS-54 and IS-136, that define various versions of digital advanced mobile phone service (D-AMPS), a TDMA system that uses a Differential QPSK (DQPSK) modulation scheme for communicating data over RF links.
Digital communication systems use a variety of linear and non-linear modulation schemes to communicate voice or data information in bursts. These modulation schemes include, GMSK, Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc. GMSK modulation scheme is a non-linear low level modulation (LLM) scheme with a symbol rate that supports a specified user bit rate. In order to increase user bit rate, high-level modulation (HLM) schemes can be used. Linear modulation schemes, such as QAM scheme, may have different level of modulation. For example, 16 QAM scheme is used to represent the sixteen variations of 4 bits of data. On the other hand, a QPSK modulation scheme is used to represent the four variations of 2 bits of data. In addition to various modulation schemes, digital communication systems can support various channel coding schemes, which are used to increase communication reliability.
Generally, channel coding schemes code and interleave data bits of a burst or a sequence of bursts to prevent their loss under degraded RF link conditions, for example, when RF links are exposed to fading. The number of coding bits used for channel coding of data bits corresponds to error detection accuracy, with higher number of coding bits providing higher bit error detection accuracy. For a given gross bit rate, a high number of coding bits, however, reduces user bit rate, since coding bits reduce the number of user data bits that can be transmitted in a burst.
The communication channel typically introduces errors in sequence. In order to improve coding efficiency, the coded bits are interleaved, before transmission. The purpose of interleaving is to distribute the errors over several code words. The term perfect interleaving is used when the sequence of the received data bits are uncorrelated. The less correlated the received data bits are at the receiver, the easier it is to recover lost data bits. On the other hand, if interleaving is not effective, large portions or blocks of transmitted data bits may be lost under degraded RF link conditions. Consequently, error correction algorithms may not be able to recover the lost data.
TDMA systems subdivide the available frequency band into one or several RF channels. The RF channels are divided into a number of physical channels corresponding to time slots in TDMA frames. Logical channels are formed from one or more physical channels, where modulation and channel coding schemes are specified. An RF link includes one or more physical channels that form the logical channels. In these systems, the mobile stations communicate with a plurality of scattered base stations by transmitting and receiving bursts of digital information over uplink and downlink RF channels.
The growing number of mobile stations in use today has generated the need for more voice and data channels within cellular telecommunication systems. As a result, base stations have become more closely spaced, with an increase in interference between mobile stations operating on the same frequency in neighboring or closely spaced cells. Although digital techniques gain more useful channels from a given frequency spectrum, there still remains a need to reduce interference, or more specifically to increase the ratio of the carrier signal strength to interference, (i.e., carrier-to-interference (C/I)) ratio. RF links that can handle lower C/I ratios are considered to be more robust than those that only can handle higher C/I ratios.
Depending on the modulation and channel coding schemes, grade of service deteriorates more rapidly as link quality decrease. In other words, the data throughput or grade of service of more robust RF links deteriorates less rapidly than those of less robust RF links. Higher level modulation schemes are more susceptible to link quality degradation than lower level modulation schemes. If a HLM scheme is used, the data throughput drops very rapidly with a drop in link quality. On the other hand, if a LLM scheme is used, data throughput and grade of service does not deteriorate as rapidly under the same interference conditions.
Therefore, link adaptation methods provide the ability to dynamically change a link protocol, which is defined by a combination of modulation scheme, channel coding, and/or the number of used time slots. The link protocol is selected based on channel conditions to balance the user bit rate against link quality. Generally, these methods dynamically adapt a system's link protocol to achieve optimum performance over a broad range of C/I conditions.
One evolutionary path for next generation of cellular systems is to use high-level modulation (HLM), e.g., 16 QAM modulation scheme, to provide increased user bit rates compared to the existing standards. These cellular systems include enhanced GSM systems with General Packet Radio Service (GPRS) extension, enhanced D-AMPS systems, International Mobile Telecommunication 2000 (IMT-2000), etc. A high level linear modulation, such as 16 QAM modulation scheme, has the potential to be more spectrum efficient than, for example, GMSK, which is a low-level modulation (LLM) scheme. Because higher level modulation schemes require a higher minimum C/I ratio for acceptable performance, their availability in the system becomes limited to certain coverage areas of the system or certain parts of the cells, where more robust RF links can be maintained.
In order to provide various communication services, a corresponding minimum user bit rate is required. In voice and/or data services, user bit rate corresponds to voice quality and/or data throughput, with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of techniques for speech coding, channel coding, modulation scheme, and for a TDMA system, the number of assignable time slots per call.
Data services include transparent services and non-transparent services. Transparent services, which have a minimum service quality requirement, provide constant user bit rates. A system that provides transparent communication services varies the gross bit rate to maintain a constant user bit rate with the required service quality. The service quality requirement of a transparent service between a mobile station and a base station is expressed in terms of a Quality of Service (QoS) vector that is defined by Equation (1): EQU QoS={R.sub.bu =X kbits/s, BER or FER&lt;Y%}, (1)
where R.sub.bu is a constant user bit rate and BER and FER are a maximum Bit Error Rate (BER) or Frame Erasure Rate (FER), respectively, and X and Y are the required user bit rate and service quality (in percentage), respectively.
Conversely, in non-transparent services, for example, GPRS, GSM's extension for providing packet data, the user bit rate may vary, because erroneously received data bits are retransmitted. Unlike non-transparent services, transparent services do not retransmit erroneously received data bits. Therefore, transparent services have a constant point-to-point transmission delay, and non-transparent services have a non-constant point-to-point transmission delay.
A communication system may provide a data service through a number of RF links supporting different combinations of channel coding, speech coding, and/or modulation schemes. For example, the system may provide a multimedia service using two or more separate RF links that separately provide audio and video signals. Under this scenario, one of the two RF links may use HLM scheme and the other link may use LLM scheme. In order to provide a constant user bit rate in a TDMA system, LLM RF link may use a higher number of time slots than HLM link.
Accordingly, in order to provide transparent data service, digital communication systems must select a suitable link protocol based on link quality, to achieve a desired service quality for a given constant user bit rate. Since link quality, e.g., C/I ratio, varies rapidly in a system, different link protocols must be used to maintain the service quality. For example, for a high quality link, less channel coding may be used to increase user bit rate. In addition to fulfilling the user bit rate and service quality requirements, it is also important to optimize system's performance in terms of minimized overall interference and/or efficient allocation of communication resources, such as the number of assigned time slots, etc.
Therefore, there exist a need for a method of selecting a link protocol for providing a transparent service in a system that supports multiple modulation and channel coding schemes, while optimizing system performance.